The present invention relates to the field of liquid crystal display devices, and more precisely to a method and a device for controlling the switching between two states of a multiplexed bistable nematic display.
Addressing a Passive Liquid Crystal Display of the STN Type
The abbreviation STN stands for “super twisted nematic”. It relates to displays having super-twisted molecule structure.
The Principle of Multiplexing—and its Limitations
Passive screens capable of displaying a large number of rows (e.g. STN technology makes it possible to obtain up to about 500 rows) use an addressing technique known as multiplexing.
With a matrix screen of medium resolution, the person skilled in the art knows that there is no question of individually connecting each pixel to an independent control electrode, since that would require one connection per pixel which is technologically impossible as soon as the screen becomes complex. It is possible to save on connections by making use of the multiplexing technique when the electro-optical effect used is non-linear, as applies to the usual liquid crystal techniques known as twisted nematic (TN) and super twisted nematic (STN). Each pixel is constituted by the intersection between a row electrode and a column electrode. The pixels are arranged in a matrix system with n groups each having m pixels. For example, there are n rows and m columns for matrix screens or n digits and m digit portions for digit displays. In sequential addressing mode, which is the mode that is in most widespread use, a single row is selected at a time. While a row is selected, column signals are applied simultaneously to all of the pixels in the row, and then the technique moves on to the following row, and so on, down to the last row. The frequency at which each row is refreshed electrically must be high enough to obtain good visual characteristics for the displayed image (about 50 times per second).
The time required for addressing the image is equal to the time required for addressing one row multiplied by the number of rows n. With that method, a mere m+n connections suffice for addressing a screen of m×n pixels, where m is the number of columns in the matrix in question. A multiplexed matrix screen is shown in FIG. 1.
The signal to which the pixel is subjected is the difference between the signal applied to the row and the signal applied to the column for which the pixel occupies the intersection.
The type of screen as shown in FIG. 1 is said to be a “passive screen”: it does not include active elements enabling the pixels to be electrically isolated from one another. A row electrode is common to all of the pixels of the row and a column electrode is common to all of the pixels of the column, without there being any active element (e.g. a transistor). As a result, passive screens are much simpler to manufacture than are active screens which include one transistor or one diode per pixel.
The drawback of multiplexing is that a pixel is addressed by column signals throughout the time the image is being addressed, and not only while its own row is being activated. That is to say, while the image is being written, a pixel on the screen receives in succession the column signals for its entire column. It can be assumed that the signals applied to the pixel outside the time during which its row is selected act as interfering signals that have an effect on the electro-optical response of the liquid crystal pixel. More precisely, for passive matrices of the TN, STN, or similar type, the state of the liquid crystal in a pixel depends almost exclusively only on the root mean square (rms) value of the voltage which is applied thereto during the image addressing time, under the usual operating conditions. As a result, the final state of the liquid crystal molecules, which means essentially the optical transmission state of the pixel, is determined by the rms voltage applied during the image addressing time. Optimizing row and column signals leads to the Alt and Plesko criterion (Alt, P. M., et al., IEEE Trans Electron Devices, ED 21: pp 146-155) which puts a practical limit on the number of rows a screen can have.
One of the principles limiting sequential addressing by one row at a time is that the voltage applied to a given pixel passes through a very clearly marked maximum each time its row is selected. The liquid crystal of the pixel then presents an instantaneous response characterized by relaxing between two occasions on which the row is addressed, i.e. between two consecutive frames. This leads to a high level of flicker and to an apparent loss of contrast. This effect is commonly referred to as “frame response”. To limit this effect, it is necessary to select a liquid crystal having a response time that is slow, to the detriment of the speed performance of the display.
Reduction of the “Frame Response” Effect by Multi-Line Addressing (MLA)
U.S. Pat. No. 5,420,604 proposes a novel addressing technique for an STN screen characterized by selecting a plurality of rows simultaneously (referred to as MLA or MLS for multi-line selection). That method relates solely to passive screens in which the optical response of the liquid crystal is a function mainly of the applied rms voltage.
By addressing a plurality of rows simultaneously, it is possible to reduce the “frame response” effect considerably, since during the frame time, the row receives not only one, but a plurality of selection pulses. It is then possible to use a liquid crystal having a fast response time.
Implementing MLA requires row selection signals to be generated that are “normalized and orthogonal”, and sometimes requires an image memory to be incorporated in the screen driver circuit. That leads to control electronics of greater expense.
Reference can usefully be made to the above-mentioned document in order to understand the kind of signals required. The term “normalized” means that the row selection signals must be normalized so that they all possess the same rms value. The term “orthogonal” means that the row selection signals must be adapted so that multiplying any one of the row selection signals by the signal for a distinct row gives a signal in which the integral over the frame period is zero.
Addressing a Bistable LCD of the Cholesteric Type (Planar-conical Focus Transition)
PCT Patent No. WO 00/74030 describes a method of addressing a plurality of rows simultaneously applied to a screen using a bistable liquid crystal having a chiral component (of the cholesteric type). In that document, rows that are addressed simultaneously must be addressed by signals that are mutually orthogonal. It is necessary to control accurately the rms voltage applied to the pixel during some of the four addressing stages of a screen based on a cholesteric liquid crystal-based screen. The use of orthogonal signals for addressing the row enables the voltages to be controlled effectively.
Description of the Bistable Screen (FIG. 2)
Recently, a new bistable display has been proposed and is described in French Patent No. 96/04447.
It is constituted by a cholesteric or chiralized nematic liquid crystal layer between two plates or substrates, at least one of which is transparent. Two electrodes are disposed on the respective substrates and serve to apply electrical control signals to the chiralized nematic liquid crystal situated between them. On the electrodes, anchor layers orient the liquid crystal molecules in desired directions. On a master plate, molecules are anchored strongly with a slight incline, while on the slave plate, anchoring is weak and flat. The anchoring of the molecules to these surfaces is monostable.
The device also includes an optical system.
The two bistable textures U (uniform or weakly twisted) and T (twisted) of the liquid crystal are stable without an applied electric field. This is obtained for a zero or small angle between the anchor direction on the master plate and on the slave plate. The twists of the two textures differ in absolute value by about 180°. The spontaneous pitch p0 of the nematic is selected to be close to four times the thickness d of the cell (p0≈4·d) in order to ensure that the energies of the textures U and T are essentially equal. With no applied field, there exists no other state with lower energy: U and T are genuinely bistable.
Switching from One Texture to the Other by Breaking the Anchoring
Physical Principle
The two bistable textures are topologically distinct, and it is not possible to transform one into the other by continuous volume distortion. Transformation from texture U to texture T, or vice versa, therefore requires either anchoring on the surfaces to be broken, as is induced by a strong external field, or else a disinclination line to be displaced. This second phenomenon which is much slower than the first can be ignored and is not described in detail below.
Any liquid crystal alignment layer can be characterized by zenithal anchoring energy Az. This energy is always finite. It can be seen shown that there then exists a threshold field Ec that is also finite (threshold for breaking the anchoring), which gives a homeotropic texture (H) at the surface regardless of the preceding texture with no applied field.
Breaking anchoring requires the application of a field that is not less than the threshold field Ec. The field must be applied for a sufficient length of time to ensure that the reorientation of the liquid crystal in the vicinity of the surface leads to the homeotropic texture. This minimum length of time depends on the amplitude of the applied field, and also on the physical characteristics of the liquid crystal and of the alignment layer.
For the static situation (fields applied for a few milliseconds or longer),
      E    c    ≈      Az                            K          33                ⁢                  ɛ          0                ⁢        Δ        ⁢                                  ⁢        ɛ            where Az is the zenith anchoring energy of the surface, K33 is the elastic bending coefficient of the liquid crystal, Δ∈ is its relative dielectric anisotropy, and ∈0 is the dielectric constant of a vacuum.
Vc is defined as the voltage for breaking anchoring such that: Vc=Ec·d where d is the thickness of the liquid crystal cell.
The anchoring is said to be broken when the molecules are normal to the plate in the vicinity of said surface, and the return torque exerted by the surface on the molecules is zero. In practice, it suffices for the difference between the orientation of the molecules and the normal to the surface to be sufficiently small, e.g. less than 0.5°, and for the torque which is applied to the molecules at the surface to be sufficiently small. When these conditions are united, the nematic molecules close to the broken surface are in unstable equilibrium when the electric field is switched off, and can return either to their initial orientation, or else turn in the opposite direction so as to induce a new texture differing from the initial texture by a twist of 180°.
The final texture is determined by controlling the waveform of the applied electrical signal, and in particular it depends on the way in which the field is returned to zero.
Lowering the voltage of the pulse progressively minimizes flow, with molecules close to the master plate descending slowly towards their equilibrium state, so that their elastic coupling with the molecules in the center of the sample causes them to incline likewise in the same direction, this movement diffusing to the slave plate where the molecules incline in turn quickly into the same direction, assisted by the surface torque. The uniform state U then builds up progressively at the center of the cell.
When the field drops suddenly, the orientation of the liquid crystal is modified, initially at the vicinity of the strong surface (master plate) with a surface relaxation time equal to
                    γ        i            ⁢              L        2              K    ,            where      ⁢                          ⁢      L        =                  K        33            Az      is the extrapolation length of the strong layer and γ1 is the viscosity in rotation of the liquid crystal. This time is typically of the order of one-tenth of a microsecond (μs).
Switching the strong surface in such a short length of time leads to a strong flow close to said surface, which diffuses into the volume and reaches the weak surface (slave plate) after a characteristic length of time that is shorter than one microsecond. The shear induced on the weak surface (slave plate) creates a hydrodynamic torque on the molecules of said surface. This torque is in the opposite direction to the elastic torque induced by the inclination of the master plate. When the shear is strong enough, the hydrodynamic torque on the weak surface is the stronger torque, thereby promoting the twisted texture T. When the shear is weaker, the elastic torque on the weak surface is stronger, and it induces the uniform texture U.
The direction of rotation of the molecules in the cell is represented by an arrow in FIG. 2.
Thereafter the volume reorients, with a characteristic volume relaxation time τvol equal to
            γ      1        ⁢          d      2        Kwhere d is the thickness of the cell. This time, which is typically of millisecond order, is much greater than the relaxation time of the strong surface.Practical Embodiment
In general, the switching of a BiNem liquid crystal takes place in two stages:
First Stage: Stage of Breaking Anchoring, Written C
The stage C consists in applying an electrical signal characterized by the fact that it breaks the anchoring on the slave plate. In general, the shorter the stage C, the greater the peak signal amplitude that needs to be applied.
For given amplitude and duration, the detailed waveform of the signal (slopes, intermediate levels, . . . ) does not have a determining effect on the behavior of the following stage, providing that anchoring is indeed broken.
Second Stage: Selection Stage, Written S
The voltage applied during the stage S must enable one of the two bistable textures U or T to be selected. Given the above-explained effect, it is the falling waveform of the electrical signal applied to the terminals of each pixel that determine switchover from one texture to the other.
The term “writing” is used arbitrarily for switching to the twisted texture T and the term “deleting” is used arbitrarily for switching to the uniform texture U.
To write a pixel, i.e. to switch its texture to T, it is necessary:
Stage C: Breaking Anchoring
To apply a pulse delivering a field greater than the field for breaking anchoring on the slave plate and to wait for long enough for the molecules to rise in the pixel. The breaking field is a function of the elastic and electrical properties of the liquid crystal material and of its interaction with the anchoring layer deposited on the slave plate of the cell. It can lie in the range a few volts to about 10 volts per micrometer (V/μm). The time required for the molecules to lift is proportional to the rotational viscosity γ and inversely proportional to the dielectric anisotropy of the material used, and also to the square of the applied field. In practice, this time can be lowered to a few microseconds for fields of about 20 V/μm.
Stage S: Selecting the Texture
Thereafter it suffices to lower the field quickly, creating a sudden drop of the control voltage in a few microseconds or at most in a few tens of microseconds. This sudden drop of voltage, of amplitude ΔV, is such that it is capable of inducing a sufficiently intense hydrodynamic effect in the liquid crystal. To produce the texture T, this drop must necessarily cause the applied voltage to switch from a value greater than the anchoring breaking voltage Vc to a value that is smaller than that. The time required for the applied field to drop is less than one-tenth its duration or less than 50 microseconds with long pulses.
FIGS. 3a1 and 3a2 show two implementations of pulses that induce the texture T.
In FIG. 3a1, the pulse comprises a first sequence of duration τ1 of amplitude P1 such that P1>Vc, followed by a second sequence of duration τ2 of amplitude P2 slightly smaller than P1 such that P2>Vc and P2>ΔV, which second sequence switches suddenly to zero. In FIG. 3a2, the pulse comprises a first sequence of duration τ1 of amplitude P1>Vc followed by a second sequence of duration τ2 and of amplitude P2 such that P2<Vc and: P1−P2>ΔV.
Stage C: Breaking Anchoring
To delete it is necessary to apply a pulse supplying a field greater than the anchoring breaking field on the slave plate and to wait long enough to allow the molecules to lift in the pixel, as when writing.
Stage S: Selecting the Texture
French Patent No. 96/04447 proposes two ways of implementing a “slow” descent, as shown diagrammatically in FIGS. 3b1 and 3b2. The delete signal is either a pulse of duration τ1 and amplitude P1 followed by a slope of duration τ2 with a descent time that is greater than three times the duration of the pulse (FIG. 3b1), or else a staircase descent in the form of a signal having two plateaus (FIG. 3b2) (first sequence of duration τ1 and amplitude P1, followed by a second sequence of duration τ2 and amplitude P2 such that either P2>Vc and P2<ΔV, or else P2<Vc and P1−P2<ΔV). The staircase descent with two steps is easier to implement with digital electronic means, so the slope is not described in detail herein. Nevertheless, it is possible to imagine devising a descent with a number of plateaus that is greater than two.
The waveforms of pulses characteristic of switching to one or the other of the textures are given in FIG. 3 (refer to French Patent No. 96/04447 and Giocondo, M., et al., “Write and erase mechanism of surface controlled bistable nematic pixel,” Eur. Phys. J. AP., Vol. 5, pp 227-230 (1999)). The durations and the voltages of the plateaus (P1, τ1) and (P2, τ2) have been determined experimentally for the examples given below.
The Multiplexing Principle Applied to the Bistable Nematic Devices (BiNem)
The BiNem screens under consideration are likewise in the form of n×m pixels (FIG. 1) each pixel being located at the intersection of two perpendicular conducive strips disposed on the two respective substrates as described above. The pixel of row N+1 and column M is shown shaded. The device has connections and electronic circuits placed on the substrate or on auxiliary cards.
The writing and deleting signals applied to the pixels are made by combining row signals and column signals. They enable the screens in question to be written and deleted row by row, i.e. quickly.
Signals must be applied to the rows and the columns such that the voltages that result across the terminals of a pixel are of a type shown in FIG. 3: the voltage applied to the pixel during the row write time must be equal to a pulse which, on request, comes to an end either suddenly, leading to a sudden drop of voltage greater than or equal to ΔV so as to create the twisted texture T (usually optically black), or else to descend progressively in steps so as to create the uniform texture U (the state which is usually optically bright).
The possibility of switching between the textures T and U and vice versa, by multiplexing, is demonstrated by the electro-optical curve given in FIG. 4: the BiNem pixel is addressed with a pulse having two plateaus having a fixed value P1 and a variable value P2. Optical transmission is given as a function of the value of the second plateau P2, with P1=16 V. The pulse durations are 0.8 milliseconds (ms). Given the orientation of the polarizers in this example, a transmission minimum corresponds to the state T and a transmission maximum corresponds to the state U.
Writing Zones
For voltages P2 greater than about 11 volts, the voltage drop at the end of the plateau 2 is sufficient for writing. For voltages P2 less than 5 volts, the voltage drop at the end of the time τ1 has written, the voltage of the plateau 2 is less than Vc, the voltage drop at its end can no longer cause the texture to switch.
The value of the voltage drop ΔV needed for writing is equal to about 10 volts, for P1=16 V and Vc=6 V.
Delete Zone
It can be seen from the curve in FIG. 4 that deleting takes place for a voltage P2E lying in the range 6 V to 9 V. In this voltage range, at the end of time τ1, the molecules close to the slave plate are entrained by the flow and thus in the write direction. During plateau 2, slightly above the breaking voltage, they become almost vertical while being slightly inclined in the delete direction because of the elastic coupling with the master plate. At the end of time τ2, the voltage drop of less than ΔV is too small for the second flow to cause the molecules to stand upright and fall in its direction, and thus write. The “slow” descent is thus implemented in two steps.
The values of the second plateau corresponding to one or other of the textures are shown in FIG. 5.
In this example, during stage C of duration τ1, a voltage P1 is applied that is suitable for breaking anchoring, and during the stage S of duration τ2, a voltage P2 is applied. The texture obtained depends on the value of P2.
Multiplexing BiNems in the Prior Art
F1 and F2 are defined as two operating points situated at the rising or falling point of inflection in the optical transmission curve of FIG. 4. We consider F2 by way of example. The voltage corresponding to the point F2 is equal to 11 V, and may correspond to the value of the second plateau A2 of the row signal. The value of the column voltage C=2 V corresponds to the voltage difference needed to obtain the pixel voltage corresponding either to texture T (minimum transmission) or to texture U (maximum transmission). The value of the second plateau applied to the pixel is then either P2I=A2+C for writing (texture U) or else P2E=A2−C for deleting (texture T) with:                for the row signal: A1=16 V; A2=10 V;        for the column signal: C=2 V;        for the signal across the terminals of the pixel: P1=16 V; P2E=8 V; P2I=12 V.        
These values vary depending on the properties of the liquid crystal and of the alignment layer, and can easily be adjusted for other screens made on the same principles using other materials. An example is given in Dozov, I., et al, “Recent improvements of bistable nematic displays switched by anchoring breaking,” Proceedings of SAID 2001, pp. 224-227 (2001).
FIG. 6 shows the principle of row and column signals for writing and deleting when above-defined operating point F2 has been selected. The row signal (FIG. 6a) comprises two plateaus: the first provides the voltage A1 during τ1, the second A2 during τ2. The column signal (FIGS. 6b and 6c) of amplitude C is applied solely during time τ2, and is positive or negative depending on whether it is desired to delete or to write. The time τ3 separates two row pulses. FIGS. 6d and 6e show the signals applied respectively to the terminals of a deleted pixel and to the terminals of a written pixel. These signals are very simple and enable all of their parameters to be adjusted easily to the characteristics of the screen.
Optimizing the Column Signal as Described in French Patent No. 02/1448
In a patent application filed in France on Feb. 6, 2002 under the No. 02/01448, the Applicant has described various improvements to displays of the BiNem type seeking to optimize the column signal. Those improvements are recalled below in order to incorporate them in the present patent application.
In that document, the parameters of the signals applied to the column electrodes of the screen are adapted so as to reduce the rms voltage of the interfering pixel pulses to a value which is lower than the Freedericksz voltage, so as to reduce the interfering optical effects of the addressing.